Designing High-Voltage PCBs with Creepage Checks in Genuine 3D
High voltages introduce physical, electrical, and economic issues. How can true 3D design, considering creepage, help you optimize your PCB for an unforgiving world?
High-voltage circuits are different. Like high-speed design, high-voltage PCB design breaks down comfortable assumptions about how things work, bringing another dimension into play. With high-speed design, you have to stop seeing connections as a given and start seeing them as components that affect function. With high-voltage PCB design, you have to do the same with insulators. Those insulators can create connections where you don’t want them, and the higher the voltage, the more likely they are to do that. These are not just considerations for PCB power supply design. They are indispensable considerations for any PCB design where creepage currents can cause issues.
What does “high voltage” really mean?
Context is important here. Do we mean high enough to be dangerous or high enough to risk a malfunction or something else? High-voltage applications are highly diverse.
The first part of that context is what the PCB is for.
Shock risk, high voltage, challenging environment
Typical applications include raw material extraction, power generation, military equipment and some medical devices. All of these are safety-critical.
Works no matter what—voltage varies and can be lower
The need for high-voltage-aware design is not always obvious. For example, a collision-avoidance system can use LIDAR. An array of avalanche photodiodes like the QA4000-10-TO from First Sensor, senses light, which is amplified and converted to digital information.
The diodes themselves require a relatively high-voltage bias. They are called “avalanche photodiodes” because they break down suddenly, but in an intentional way. A small stimulus sends them over the edge. A relatively high voltage is required to take them close to that edge.
FR4 can break down even at relatively high insulator thickness, especially when it has had time to age. And the lower FR4 thicknesses used in multilayer PCBs break down very easily. With just a 1080, 2116, or 7628 layer separating them, the insulation between conductors on two PCB layers will break down at under 3kV. The final thickness of each insulating layer is also affected by PCB manufacturing processes. Manufacturers often provide useful information that helps to make your insulating layer choice as scientific as possible, such as this information on standard build-ups from multi-cb.
eCADSTAR does not choose your materials or layer thicknesses, but it does check in 3D, and even across layers, for sufficient clearance, to any rules you care to apply.
Creepage is distinct from 2D clearance. The length of a creepage path is always greater than or equal to the 2D clearance. Imagine you are on an Antarctic expedition. You are following the group when you hear a crack. A crevasse opens in front of you. There is only 10 meters between you and the group, but to reach them, you have to go around the end of the crevasse.
For an Antarctic expedition, the extra distance to travel over this “creepage path” can be unfortunate, but on a PCB, we can play a longer electrical path to our advantage, to increase layout density. These images from eCADSTAR show how cutting and extending a slot in a PCB fixes a creepage error, saving precious real-estate on a high-density circuit board.
That just looks like a 2D check, so where does 3D come in? Well, the real world is 3D and it’s hard to work with it unless you visualize in 3D too.
Electric current can find creepage paths that you would not necessarily notice or detect in 2D. Even if you detect them, it’s much harder to see what’s going on, fix problems and optimize.
Current can creep just as easily from layer to layer as it can on the same layer. Current finds electrical paths wherever they are. In this case, there is a path from top to bottom layer, between 90V and -5V, that traverses the inner surface of a hole in the PCB.
Working to standards is fundamental to high-voltage PCB design. The variables are many and they can be more subtle than they appear. Standards have been consolidating recently, but there are still many of them, including IPC-2221A, UL609050-1, UL-61010-1 and the more recent IEC 62368-1. Different creepage clearances apply at different ranges of voltage difference.
Perhaps the most basic formula in Electronics is I=V/R
An insulator such as a PCB laminate has a very high resistance, but
- The higher V becomes, the higher R has to be to provide the same defense against creepage current
- The more polluted the surface of a PCB becomes—wetter, oilier, dustier, etc.—the lower its resistance becomes
- The older an insulating material becomes, the more it degrades, and the lower R becomes, with ageing accelerated by harsh environmental conditions
This is why applicable standards codify acceptable creepage clearances in terms of voltage difference, pollution levels and materials. Some of those standards are external to any individual design or range of designs. You can pick rules that apply to your materials and environment. Rules often comprise a table of 3D creepage clearances that relate to a list of voltage difference ranges. There may be several different voltages that must conform to creepage rules, even within a single design. For efficient working, you have to be able to enter your chosen rules in a clear and flexible way.
Some more subtle things to consider
Meaning of “voltage”
The definition that matters here is the potential difference between two points. But a GND (or Common) signal is not necessarily 0V and it is not necessarily the same as Earth Ground. To complicate this even further, there can be multiple grounds at different potentials with respect to Earth Ground or even Floating Grounds that that have no defined relationship to Earth Ground.
Voltage differences can only be evaluated between signals that share a common reference at some level.
AC and phase
You can evaluate spacing and creepage for AC signals too. In this case, both the specific AC voltages and their phase are key considerations. Some standards are stated in terms of RMS, but you can also evaluate in terms of peak voltage.
Formula-based rules mean you can work more dynamically
Instead of specifying a fixed minimum clearance for any part of the voltage difference rules, you can choose to express it as a formula. This boosts dynamism in clearance within a voltage range and can help you to increase density at the margins.
Replacing a fixed requirement of 0.1mm clearance within this voltage range with a formula has created a smaller allowable clearance at the low end and a larger one at the high end. This is displayed automatically in the graph on the right.
(Please note: The colored lines on the graph at the head of the arrow have been enhanced in this image to improve readability). The ability to specify a formula gives a dynamic capability that goes beyond purely physical PCB trace spacing calculations and even beyond range-based high-voltage spacing rules.
Finally—flexibility and rule re-use are key to success
Rule stacks provide more flexibility compared to single rule settings. That way, you can choose to either apply them according to the highest-priority level or apply the most conservative rule. You have to be able to mix rules defined in standards with your custom design requirements and preserve those settings in your library. Clearance and creepage rules for PCB assembly are essential to reliability, even in non-safety-critical applications. You can optimize your designs beyond the requirements of safety standards.
I hope you enjoyed this brief summary of some high-voltage PCB design issues and some of the things eCADSTAR can do to make handling them easier. There’s a lot more, but that’s what application HELP is for.
Jane Berrie•Signal Integrity Expert, Zuken Tech Center, Bristol.Jane Berrie has been involved in EDA for PCB signal integrity since the 1980s. Her articles have appeared in many publications worldwide - too many times to mention. Jane is also a past session chair for 3D IC design at the annual Design Automation Conference. Jane’s also an innovator with a unique perspective, who constantly works on new solutions in the fast-evolving world of electronic design. In her spare time, Jane has organized themed charity events - including two in aid of lifeboats and red squirrel survival. Jane is also a regular disco-goer.